In some wireless communication systems, single band array antennas are employed. However in many modern wireless communication systems network operators wish to provide services under existing mobile communication systems as well as emerging systems. In Europe GSM and DCS1800 systems currently coexist and there is a desire to operate emerging third generation systems (UMTS) in parallel with these systems. In North America network operators wish to operate AMPS/NADC, PCS and third generation systems in parallel.
As these systems operate within different frequency bands separate radiating elements are required for each band. To provide dedicated antennas for each system would require an unacceptably large number of antennas at each site. It is thus desirable to provide a compact antenna within a single structure capable of servicing all required frequency bands.
Base station antennas for cellular communication systems generally employ array antennas to allow control of the radiation pattern, particularly down tilt. Due to the narrow band nature of arrays it is desirable to provide an individual array for each frequency range. When antenna arrays are superposed in a single antenna structure the radiating elements must be arranged within the physical geometrical limitations of each array whilst minimizing undesirable electrical interactions between the radiating elements.
US 2003/0052825 A1 describes a dual band antenna in which an annular ring radiates an omni-directional “doughnut” pattern for terrestrial communication capability, and an inner circular patch generates a single lobe directed towards the zenith at a desired SATCOM frequency.
WO 99/59223 describes a dual-band microstrip array with a line of three low frequency patches superposed with high frequency crossed dipoles. Additional high frequency crossed dipoles are also mounted between the low frequency patches. Parasitic sheets are mounted below the crossed dipoles.
Guo Yong-Xin, Luk Kwai-Man, Lee Kai-Fong, “L-Probe Proximity-Fed Annular Ring Microstrip Antennas”, IEEE Transactions on Antennas and Propagation, Vol. 49, No. 1, pp 19-21, January 2001 describes a single band, single polarized antenna. The L-probe extends past the centre of the ring, so cannot be combined with other L-probes for a dual-polarized feed arrangement.